1. Field of the Invention
The present invention relates to an image forming apparatus of electrophotographic process, such as a copying machine or a printer.
It particularly relates to an image forming apparatus utilizing a photosensitive member of a high capacitance having a capacitance (C/S) per unit area satisfying a relation C/S≧1.7×10−6 (F/m2)
2. Description of the Related Art
In the image forming apparatus of electrophotographic process, a higher image quality, a higher output speed and a higher stability for the output image are being requested along with the advancement toward the full-color configuration and the digital structure, and an entry into the printing market is expected. However, in order that the copying machine and the like can enter the printing market, an image quality, an output speed and a stability even higher than the current level are essential issues to be solved.
An image bearing member currently employed in the electrophotographic process (electrostatic process) includes a selenium-based photoreceptor, an amorphous silicon photoreceptor, and an organic photoreceptor (OPC).
Among these, the amorphous silicon photoreceptor, being provided with a charge generation layer in the proximity of a surface layer as shown in FIG. 1 schematically illustrating a layered structure, can suppress the diffusion of a charge generated at an exposure. It is therefore capable of realizing a high image quality, excellent in dot reproducibility, and is excellent in durability because of a very high hardness, thus being anticipated for an image bearing member capable of realizing a high speed and a high stability (for example cf. Japanese Patent Application Laid-open No. 2004-279902).
In an image forming apparatus for forming an electrostatic latent image by a digital exposure, which exposes an image in accordance with an image signal, the latent image forming method for tone reproduction can be generally classified into following two methods.
A first method is an image processing method such as a dither method or a density pattern method, classified as a binary recording method. In the dither method, as shown in FIG. 2A, one pixel in the read input signal is made to correspond to one pixel of binary recording. In the density pattern method, as shown in FIG. 2C, one pixel in the read input signal is made to correspond to plural recording pixels. In an intermediate method of these two methods, as shown in FIG. 2B, one pixel in the read input signal is made to correspond to a partial matrix (1×1) within an m×m matrix. In such correspondence to the partial matrix, a case of 1=1 corresponds to the dither method while a case of 1=m corresponds to the density pattern method, and an arbitrary value may be adopted for varying the output image size. Also these image processing methods are classified into a screen pattern method utilizing a screen structure, and an error diffusion method not utilizing a screen structure.
A second method is a multilevel recording method, utilizing a pulse width modulation or an intensity modulation. In such method, an irradiation time or an intensity of a laser beam is modulated according to the image signal, thereby executing an area coverage modulation of a dot formed by a beam spot for each pixel. In this manner, a tone reproduction is made possible without deteriorating the resolution. It is also possible to secure the number of tone levels by combining such dither method and the pulse width modulation.
Now, there will be explained, in an image forming apparatus utilizing an image exposure with a laser scanning exposure means as the digital exposure means, a method of reproducing a medium tone portion (medium tone image portion) to a solid portion (solid image portion) by a pulse width modulation. The surface of the image bearing member (photosensitive member) is charged to a predetermined dark potential by a charger, and is subjected to an image exposure by a scanning with a laser beam. In an exposed portion of the image bearing member, the potential is decayed from a dark potential to a light potential thereby forming an electrostatic latent image on the image bearing member. Such electrostatic latent image is developed as a toner image by a reversal developing device.
In a solid image portion, the laser continues to be turned on, as illustrated in (1-3) in FIG. 3, at the latent image formation by a laser scanning exposure, and a flat latent image as shown in (2-3) is formed into a flat toner image as shown in (3-3).
On the other hand, for a medium tone, the laser oscillation time in the solid image portion is divided into 256 oscillation times (00h-FFh) in order to obtain 256 tone levels (cf. (1-1) and (1-2)). A latent image in such state involves irregularities as shown in (2-1) and (2-2), and is developed as a toner image as shown in (3-1) and (3-2).
In this manner, 256 tone levels from the solid image portion to the medium portion are reproduced.
A developing device, for developing an electrostatic latent image with a toner, is classified into two types. The first is a two-component developing device, which executes the development with a two-component developer formed by a magnetic carrier and a non-magnetic toner. The second is a one-component developing device, which executes the development utilizing toner (magnetic or non-magnetic) only as the developer.
The two-component developing device is provided with a developer carrying member, generally including therein a magnet roller formed by a magnetic member having plural magnetic poles. The two-component developer, formed by mixing and charging toner and carrier, is borne on the developing carrying member by a magnetic force, and is carried to a development area provided between the developer carrying member and the image bearing member opposed thereto. A development bias is applied to the developer carrying member to generate a developing electric field in the developing area, thereby separating the toner, sticking to the carrier, from the carrier and executing the development of the electrostatic latent image on the image bearing member. It is known that the developing method by the two-component development can provide a stable charge on the toner, because the charging is executed by mixing/agitation of the toner and the carrier, thereby providing a stable satisfactory image.
On the other hand, in the one-component developing device, a developer carrying member, carrying the toner, is contacted with the image bearing member or is opposed in a non-contact state thereto and in a proximity thereof, and a development bias is applied to the developer carrying member, whereby the electrostatic latent image on the image bearing member is developed with the toner.
However, the amorphous silicon photoreceptor (hereinafter represented as α-Si photoreceptor) involves a drawback of causing an image defect, called “blank area”. Such “blank area” will be explained in the following. The blank area means, when a medium tone portion in a leading part and a solid image portion in a trailing part, in the traveling direction of the image bearing surface, are developed as shown in FIG. 4, a phenomenon that the developed density decreases in a boundary portion between the medium tone portion and the solid image portion.
A generating mechanism for such blank area is as follows. FIG. 5 shows latent image potentials in the medium tone portion and the solid image portion. When the development bias is applied to the developer carrying member, with respect to a latent image potential (light potential) Vl of the solid image portion, a difference between a DC component Vdc of the development bias and the light potential Vl constitutes a development contrast Vcont. The development in the solid image portion is executed in such a manner that a potential, generated by the development of the latent image in the solid image portion with the toner (such potential being hereinafter called a charge potential ΔV) fills in the development contrast Vcont. Such charge potential means, with respect to the light potential, a surface potential of the toner layer after the development of the latent image in the solid image portion.
However, when there is generated a “deficient charging” in which the development is terminated in a state where the charge potential ΔV of the solid image portion cannot sufficiently fill in the development contrast Vcont as shown in FIG. 6, a potential difference is generated between the medium tone portion and the solid image portion. Such potential difference generates a “wraparound electric field” directed from the medium tone portion toward the solid image portion. Therefore, the electric field between the developer carrying member in the developing device and the image bearing member includes not only the electric field in a direction from the developer carrying member toward the image bearing member but also a wraparound electric field generated in the boundary area between the medium tone portion and the solid image portion. In the boundary area between the medium tone portion and the solid portion, the toner is not deposited on the medium tone portion but on the solid portion by such wraparound electric field, thus generating a “blank area”.
As explained above, the blank area is generated in case of a large potential difference between the medium tone portion and the solid portion, by a deficient charging in the solid portion. Therefore, even in a case, opposite to the situation shown in FIG. 4, where a solid portion in a leading part and a medium tone portion in a trailing part, in the traveling direction of the image bearing surface, are developed, the blank area is generated by the potential difference between the solid portion and the medium tone portion when the solid portion shows a deficient charging. However, in the case that a medium tone portion is developed in the leading end as shown in FIG. 4, the potential difference becomes larger because the medium tone portion is developed while the solid portion is not yet developed with the toner, whereby the blank area appears conspicuously. On the other hand, in the case that a solid portion is developed in the leading end, the development of the medium tone portion starts after the development of the solid portion; whereby the potential difference becomes smaller and the blank area becomes less conspicuous.
A charge potential, generated by the development of a latent image with the toner, is theoretically represented by an equation 1.
                              Δ          ⁢                                          ⁢          Vth                =                                            Δ              ⁢                                                          ⁢              Vt                        +                          Δ              ⁢                                                          ⁢              Vc                                =                                                    dt                                  2                  ⁢                                      ɛ                    0                                    ⁢                                      ɛ                    t                                                              ⁢                              〈                                  Q                  S                                〉                                      +                                          dm                                                      ɛ                    0                                    ⁢                                      ɛ                    m                                                              ⁢                              〈                                  Q                  S                                〉                                                                        Equation        ⁢                                  ⁢        1            
wherein:    dt: height of toner layer on image bearing member    dm: film thickness of image bearing member (total film thickness excluding a base)    Q/S: toner charge amount per unit area on image bearing member    ε0: vacuum permittivity    εt: permittivity of toner layer    εm: relative permittivity of image bearing member.
These items are substituted naturally with such units as to satisfy the dimension of the equation. In the equation 1, a first term is a potential ΔVt generated by the toner layer itself in the proximity thereof, and a second term is a potential ΔVc generated by a capacitor effect between the toner layer and the base layer of the image bearing member. A sum of both terms constitutes a potential generated at the development of the latent image with the toner, namely the charge potential ΔVth. ΔV is a measured value of the charge potential, and ΔVth is a theoretical value of the charge potential (derived form the equation 1). Also the film thickness dm of the image bearing member indicates a thickness of an actual photoconductive layer i.e. a layer thickness excluding a base 5 in the image bearing member. More specifically, in case of an α-Si photoreceptor constituted of a surface layer 1, an electron blocking layer 2, a charge generation layer 3, a hole blocking layer 4 and a base 5 as shown in FIG. 1, the film thickness dm of the photoconductive layer is a sum of the thicknesses of the surface layer 1, the electron blocking layer 2, the charge generation layer 3, and the hole blocking layer 4 excluding the thicknesses of the base 5. On the other hand, for an organic photoreceptor, the film thickness dm of the photoconductive layer is a sum of a surface layer, a charge generation layer, and a charge transport layer when there is the surface layer, and a sum of a charge generation layer and a charge transport layer when there is no surface layer. In a case where an undercoat layer is provided on the base, the thickness dm of the photoconductive layer does not include the thickness of the undercoat layer.
As the α-Si photoreceptor has a permittivity εm of about 3 times in comparison with the organic photoreceptor, the capacitance C/S (=ε0×εm/dm) per unit area for a same film thickness becomes about 3 times larger. Also for improving the dot reproducibility, it is preferable to reduce the film thickness dm of the image bearing member, thus suppressing the charge diffusion. It is found preferable to maintain the film thickness dm at 50 μm or less in the α-Si photoreceptor for realizing an acceptable dot reproducibility, and, in the organic photoreceptor, to maintain the film thickness at 17 μm or less for realizing a dot reproducibility comparable to that in the case of α-Si photoreceptor. In such case, the capacitance C/S per unit area was 1.7×10−6 (F/m2) for each image bearing member.
When the image bearing member is given a high capacitance in order to improve the dot reproducibility, even when the latent image is developed with a toner of a same Q/S (toner charge amount per unit area), the second term decreases because of the larger capacitance of the image bearing member.
Stated differently, in an image bearing member of a high capacitance such as an α-Si photoreceptor, more specifically in an image bearing member having a high capacitance per unit area (C/S) satisfying a condition C/S≧1.7×10−6 (F/m2), the charge potential ΔV decreases in comparison with an image bearing member of a low capacitance such as an organic photoreceptor. It is therefore liable to cause a “deficient charging” in which the charge potential cannot fully fill in the development contrast Vcont.
Thus, in order to improve the image quality (dot reproducibility), it is necessary to suppress the diffusion of the charge generated in the charge generation layer of the image bearing member at the exposure. The α-Si photoreceptor, in which the charge generation layer may be formed close to the surface layer as shown in FIG. 1, can suppress the charge diffusion, thus it is advantageous in providing an excellent dot reproducibility. On the other hand, in the organic photoreceptor, in which the charge generation layer is distanced from the surface layer, the level of diffusion is aggravated according to the distance. It is therefore inferior in the dot reproducibility. It is found that an acceptable dot reproducibility can be realized, in a case of an α-Si photoreceptor, for a film thickness dm of 50 μm or less, and, in a case of an organic photoreceptor, for a film thickness dm of 17 μm or less. In such state, C/S (=ε0εm/dm)=1.7×10−6 (F/m2) is selected as a lower limit. Based on these technical reasons, the aforementioned condition C/S≧1.7×10−6 (F/m2) is selected.
The capacitance is determined by the permittivity and the film thickness of the image bearing member, and varies significantly depending on the product. As the α-Si photoreceptor has a permittivity of ÷10 while the organic photoreceptor has a permittivity of ÷3, the capacitance in the α-Si photoreceptor is about 3 times that of the organic photoreceptor for a same film thickness.
Now a method of measuring the capacitance (C/S) of the image bearing member, employed in the present investigation, will be explained. A flat-shaped photosensitive plate is prepared by forming a layered structured same as the actual photoconductive layer (charge generation layer, charge transport layer and surface layer) on a metal base, and an electrode, smaller than the photosensitive member, is contacted. A current, generated when a DC voltage is applied to the electrode, is monitored and an obtained current curve is integrated in time to determine a charge amount q accumulated in the photoconductive layer. This measurement was repeated for different DC voltages, and a capacitance (C) of the photosensitive plate is determined from a gradient between the charge amount q and the voltage V. Then a capacitance per unit area (C/S) was determined based on an area (S) of the used electrode. Also a method of measuring the film thickness (dm) and relative permittivity εm of the image bearing member, employed in the present investigation, will be explained. Thickness of the aforementioned photosensitive plate was measured with a film thickness meter before and after providing the photoconductive layer, and the film thickness dm of the photoconductive layer was determined from the difference in the thickness. Also the relative permittivity εm was determined by substituting the capacitance (C/S) and the film thickness (dm), determined as explained above, into a theoretical equation (εm=(C·dm)/(S·ε0)).
As explained above, an image bearing member of a high capacitance, such as an α-Si photoreceptor, involves a drawback of causing a blank area image defect by the deficient charging described above.
Now there will be explained a drawback in tone reproducibility caused by such deficient charging. In a prior organic photoreceptor of a low capacitance, a charging of filling in the development contrast Vcont with the charge potential ΔV is completed within the developing area, whereby the development of the latent image is completed properly. On the other hand, in an image bearing member of a high capacitance, the development of the latent image cannot be completed properly because of a deficient charging, whereby the developed toner amount increases. For this reason, the development contrast Vcont, required for obtaining the necessary image density, becomes decreased and the V-D curve (showing image density as a function of development contrast at arbitrary 16 tone levels within 256 tone levels) tends to become steeper as shown in FIG. 22, whereby the tone reproducibility becomes difficult to realize.
A shift to a higher capacitance in the image bearing member is unavoidable in the trend hereafter toward a higher image quality and a higher stability. In order to utilize an image bearing member of a high capacitance such as an α-Si photoreceptor, it is essential to solve the image defect described above, namely a deficient charging in the image bearing member of high capacitance.